So in order to be able to control a gene, not only inactivate it in every cell but to be able to control the gene in specific cells at a particular time in a mouse, a technique that is used is to combine essentially homologous recombination with what's called site-specific recombination. These are molecular scissors, what, a site-specific recombination recognizes so this is our gene of interest and we flank it by homologous recombination gene targeting with two sequences which are called loxP. And if we now, and if we introduce into this cell an enzyme called Cre, which is the site-specific recombination, what happens is these sequences line up. So here we have the DNA and it goes around like that, and here are the loxP sites.